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100 Coordination Chemistry Reviews, 75 (1986) loo-127 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
Chapter 2
ELEMENTS OF GROUP 2
Peter Hubberstey
2.1 INTRODUCTION..............................._....... 101
2.2 METALS AND INTERMETALLIC COMPOUNDS................. 101
2.2.1 Structural and Thermodynamic Properties ........ 101 2.2.2 Hydrogen Storage Applications .................. 102
2.3 SIMPLE COMPOUNDS OF THE ALKALINE EARTH METALS ...... 106
2.3.1 Binary Derivatives ............................. 106 2.3.2 Ternary Derivatives ............................ 107
2.4 COMPOUNDS OF THE ALKALINE EARTH METALS CONTAINING ORGANIC MOLECULES OR COMPLEX IONS .................. 109
2.4.1 Complexes of Significance in Bioinorganic Chemistry ...................................... 109
2.4.2 Complex Formation in Solution .................. 113 2.4.3 Beryllium Derivatives .......................... 113 2.4.4 Magnesium Derivatives .......................... 117 2.4.5 Calcium Derivatives ............................ 120 2.4.6 Strontium and Barium Derivatives ............... 121
REFERENCES.... _,.............I........................... 124
OOlO-8545/86/$9.80 0 1986 Elsevier Science Publishers B.V.
101
2.1 INTRODUCTION The format adopted previously' for reporting the chemistry of
these elements has been retained for the present review, the abstracted data being considered in sections which reflect topics currently of interest and importance. Some topics (eg. complexes of crowns, cryptands and related molecules) are common to Group 1 and Group 2 elements; for these the published data are discussed in the appropriate section of Chapter 1. the Group 2 elements are covered in this
The topics unique to Chapter.
2.2 METALS AND INTERMETALLIC COMPOUNDS Once again a significant number of the papers abstracted for
this section involve the application of alkaline earth metals and their intermetallic derivatives (eg. Mg2Ni, Mg2Cu, CaNi5) as hydrogen storage materials: to give appropriate coverage to this subject, the section has been subdivided as for the previous review. 1
2.2.1 Structural and Thermodynamic Properties Reanalysis of the X-ray data from the high pressure phase Be(II)
has revealed an alternative to the previously accepted hexagonal cell (a = 432.8, c = 341.6pm}; 2 it is a smaller orthorhombic cell (a = 216.8, b = 375.5, c = 341.6pm) with four Be atoms occupying the 4c positions (x-0.67, y-0.82, z-0.70) of the P2,2,2 space group. Activity data for liquid Mg-X (X = Si,Ge,Sn,Pb) binary systems
have been calculated fron the corresponding liquidus and vapour pressure data. 3 They are used to develop models for the structure of the liquid close to the Mg2X composition; the most realistic scheme is that in which successive dissociation of the compounds occurs with the formation of the intermediate associate:
Mg2X-bMg + MgX-+Mg + X (X = Si,Ge,Sn,Pb) .-.(11
Collins et al have determined the structures of Be17Zr2, 4
Be,6Re0.925 and Be12Mo. 6
high pressure (300 x 106Pa) Schafereta17 have prepared BaA12 under - high temperature (1273K) conditions
and, after quenching to ambient conditions, have determined its structure as a metastable phase. Pertinent crystallographic parameters for all four intermetallic compounds are collated in
102
Table 1.
Single crystal XRD data have been used to assess the charge
density distribution in the Laves phases MgX2 (X = Cu,Zn). 8
Electron transfer from Cu to Mg and from Mg to Zn was observed;
difference Fourier syntheses also show that there are residual
electrons at the centre of the Cu4 or Zn4 tetrahedra.
The electronic structures of Mg3X2 (X = Sb,Bi) have been
calculated using an extended Hcckel technique to facilitate
interpretation of soft X-ray emission and X-ray photoemission
spectroscopic data. 9
Single crystal XRD structural data have been accumulated for a
number of ternary intermetallics; IO-15 the compounds studied are
listed in Table 1 together with pertinent crystallographic
parameters. The ternary borides investigated by Jung 10 are
members of the homologous series MnRh3n_lB2n (M = Ca,Sr) whose
structures are built up by combining elements of the MRh3B2 and
MRh2B2 (M = Ca,Sr) structures; viz., M2Rh5B4 (M = Ca,Sr), M3Rh8B6
(M = Ca,Sr), Sr5Rh,4B,0 and Ca7Rh20B,4 are l/l, 2/l, 4/'l and 6/l
combinations, respectively. The structures of the ternary
aluminides wereascertained by Schafer et al 11-13 and that of
Mg28.4CU57.9Si13.7 by a group of Japanese chemists; the structure
of the latter is a variant of that of the Laves Phase of
composition, Mg2Cu3Si.
2.2.2 Hydrogen Storage Applications
lmamura et alI6 have continued their investigation, first
reported in the 1983 review, 17 of the hydriding characteristics
of magnesium metal clusters formed by cocondensation of magnesium
atoms and thf mol.ecules at 77K. The results of hydrogen-
deuterium isotope scrambling and surface modification experiments
for both Mg-thf mixtures and Mg powder indicate that the higher
activity of the former substrate is attributable to a rapid
susface process. 16
Hydrogenation of MgC12 suspensions in thf in the presence of a
CrC13-C14H,0Mg catalyst has also been studied; 18 the products
obtained incl.ude MgHC1, MgHCl,MgC12 and MgHCl,MgH2.
Both theoretical 19 and experimental 20-23 investigations of the
hydrogenation ofmagnesium-containing alloys have been completed.
The results of model calculations, based on extended Huckel MCI
theory, for the interaction of hydrogen atoms with a variety of
103
Table 1. Crystallographic parameters for diverse intermetallic compounds containing an alkaline earth metal.
Phase Symmetry Space Group
a/pm b/pm c/pm Ref.
Be17Zr2 trigonal R3m 753.8 - 1101.5 4
Be16Reo . 92 cubic F43m 587.5 - 5
Be12Mo tetragonal 14/mmm 725.1 - 423.4 6 BaA12 cubic Fd3m 870.2 - 7
CaRh3B2 hexagonal PG/mn~n 555.1 - 291.9 10
M0.66Rh3B2 (M=Sr,;a)* hexagonal P6/mmm 566.8 - 281.5 10
M2Rh5B4 (M=Ca,Sr) orthorhombic Fmmm 546.0 972.7 1122.0 10
M3Rh8B6 (M=Ca ,Sr)* orthorhombic !?mmm 550.0 969.8 1704.7 10
SrsRh14B10 orthorhombic Fmmm 560.1 984.5 2890.3 10
Ca7Rh20B14 orthorhombic Fmmm 553.0 965.2 4042 10
CaCr2Al10 tetragonal P4/nmm 1295.7 517.9 11
Ca4Cr7A148 trigonal P3ml 1028.1 - 1157.9 12
CaCu4A18 tetragonal 14/mmm 882.3 515.1 11
Ca3Cu2A17 trigonal RSm 561.4 - 2585.3 11
CaZn2A12 tetragonal 14/mmm 412.7 - 1167.1 13
Mg28.4CU57.9Si13.7 cubic P4132 695.98 - - 14
Sr6A12Sb6 orthorhombic Cmca 2041.0 690.0 1350.1 15
* Several isostructural compounds were synthesised; the crystallographic data refer to the element listed first.
Mg3mg3 clusters (I) have been presented: 19 - they serve as a basis
for qualitative speculation on the rates of hydriding and dehydriding of magnesium alloys and on the thermal properties of
WI Mg M = Mg, Al, Ti, V, :'.
~-_-~~~g-__-M____&g__~ Cr, Mn, Fe, Co, Ni, Cu.
: . : *
The Mgpfg3 cluster showing axial approach of two hydrogen atoms.
104
doped magnesium hydrides. Those clusters containing Mg Ti V Cr
Mn and Fe were found to be stable (with respect to the isolated
atoms), while clusters containing Co Ni and Cu were unstable in
this particular geometry (1). The presence of a second metal
invariably decreased the work function of the Mg cluster: the
introduction of hydrogen, however, increased its work function. 19
The interaction of magnesium-copper 20 and magnesium-nickel 21
alloys containing hydride forming metals with hydrogen has been
studied using XRD techniques; the initial phase compositions of
the alloys and those of the hydrided products are summarised in
Table 2. The introduction of these metals into the magnesium
matrix invariably has a significant catalytic effect on the
hydrogenation process. 20,21
Table 2. Phase compositions of magnesium alloys before and after
hydrogenation. 20,21
Alloy Initial Phase composition Final Phase composition*
Mg-Cu-Ca Mg2Cu, Mg2Ca, Mg MgCaHq, MgHz, CaH2, M&u2
Mg-Ni-Ca Mg2Ni, W2Ca, Mg MgCaH*, Mg2NiHq, MgH2
t Mg-Ni-Sc Mg2Ni, ScNi2, Mg Mg2NiH4, MgH2
Mg-Ni-Y Mg2Ni, YNi2, Mg MgZNiHq, MgH2, YH2
Mg-Ni-Ce Mg2Ni, W12Ce, Mg Mg2NiH4, MqH2, CeH3
* For some samples the products also contained some or all of the reactant phases; only novel phases are included here.
t Positive evidence for neither ScH3 nor ScNi2Hx was obtained.
The kinetics of the hydrogenation of Mg2Ni 22 and of the
Mg-Mg2Cu eutectic alloy (xcu = 0.145) 23 have been investigated.
For Mg2Ni, the rate-determining step was found to be
dissociative chemisorption on nickel, involving heat transfer as a
rate-delaying step. 22
The hydriding characteristics of CaNi have been the subject of
both a thermodynamic 24 and a spectroscopic 25 analysis. Precise
pressure-composition isotherms have been determined 24 for the c1-
and a' -phases of the CaNiS-H2(D2) systems. Interpretation of the
105
results suggests that the hydrogen atoms are situated in a single type of interstitial site (maximum occupancy = 0.5H/CaNi5) in the a-phase and in two types of interstitial site (maximum occupancy = l.OH/CaNi5 and 2.0H/CaNi5) in the al-phase. 24 Electron spectroscopy (Auger electron, X-ray photoelectron and U.V. photo- emission) studies 25 of CaNi 5 and the related Haucke compounds CaxEul_xNi5 (x = 0.17, 0.75) indicate that their widely different hydsiding characteristics are probably not dominated by electronic contributions (the band structures of these materials are almost identical) but are influenced by an increase in the size of the interstitial sites with increasing x and by the variation in the permeability of the passivation films formed on the surfaces of these compounds. 25
Single crystal XRD studies have been performed for Mg2FeH6 26
and Mg2NiH4_ 27-29 Prepared by a sintering technique at 773K and under (2-12) x 106Pa of hydrogen, Mg2FeH6 has a cubic (Fm3m) structure (K2PtC16-type) with a0 = 644.3pm. 26
Aspects of the structural chemistry of Mg2NiH4 (Mg2NiD4), ascertained from X-ray and neutron diffraction data, have been reported by three independent groups; 27-29 although broadly in agreement some of the details in the three papers differ slightly. Mg2NiH4 is polymorphic, existing in monoclinic, orthorhombic and cubic modifications. The pure monoclinic phase, which is the stable ambient temperature phase, has been prepared by hydriding a virgin sample of Mg2Ni. 27 It converts irreversibly to the cubic modification at 518K 27 (508K)28 and ambient pressure (105Pa). The third, orthorhombic, phase often contaminates samples of the monoclinic modification; 28 the experimental conditions required to synthesise a pure orthorhombic phase are unknown. 27 The orthorhombic phase transforms readily to the cubic polymorph at 508K and ambient pressure (105Pa) 27 and converts to the monoclinic phase on grinding at ambient temperature (298K) and pressure (105Pa):28 it is stabilised by 10% substitution of Ni by Co. 28
The monoclinic structure can be refined in either Cc space group with a = 1503.7, b = 641.1, c = 649.3pm, 8 = 118.8 02' or C2/m space group with a = 1320.1, b = 640.7, c = 649.1, B =
93.21028 (a = 649.6, b = 641.2, c = 660.2pm, B = 93.23°).2g The main peaks of the orthorhombic phase can be indexed with a = 657.2, b = 452.0, c = 456.0pm; 28 the presence of several weak peaks, however, indicates that longer range ordering exists and
106
that the true unit cell is somewhat larger. The cubic
be refined in Fm3m space group with a = 653.0pm. 28
The monoclinic polymorph consists of a 3-D network of
phase can
distorted 7-l
MgH6 octahedra with shared vertices (average r(Mq...H) = 180pm) iL'
the hydrogen atoms form square planar coordination spheres around
the Ni atom (average r(Ni...H) = 156pm). 29 It can be considered
to be constructed from alternating CaTi03- and Re03-type units.
In the cubic polymorph, the metal atoms farm a ccp CaF2-type
structure. At T = 513K, most of the hydrogen atoms are
distributed among the 48h sites (x,x,0) [x-0.2] for which the
calculated Mg-H contact is 166pm; such a distance suggests a
preferential Mq-H bond. At T>513K, the hydrogen atoms show no
preference for 48h or 24e (x,0,0) sites. 27
The electronic structures of the ternary hydrides, Mg2FeH6, 30
Mg2NiH4 31 andCa2RuH6 30 have beenstudiedusing the augmented plane
wave band structure method. Whereas the A2BH6 compounds were
found to be semi-conductors, 3o found to be an
insulator. 31 Mg2NiH4 was
2.3 SIMPLE COMPOUNDS OF THE ALKALINE EARTH METALS
The decrease in interest in these compounds noted in the 1983
review,32 has not yet been reversed. Indeed so few papers have
been abstracted this year that the various subsections included
previously have been amalgamated under the two broad subject
headings of 'binary derivatives' and 'ternary derivatives'. To
avoid duplication with other Chapters of this review, the ternary
compounds considered do not include those which contain a metal
from the p-block of the Periodic Table.
2.3.1 Binary Derivatives
As for previous reviews, those papers in which the catalytic
activity of alkaline earth metal oxides is described, although
numerous, are not considered here since their content is of but
marginal interest to the inorganic chemist.
Electronic and geometric structures and physicochemical
properties of MO, (MOH)+ and MOH (M = Be,Mg) have been
calculated by ab initio SCF MO methods; pertinent data are
summarised in Table 3.
Extensive solid solutions have been observed in the Sr12-BaI2
system both at ambient and higher (2.0GPa) pressures. 34 The
107
Table 3. Structural properties for MO (MOH)+ MOH (M = Be,Mg) molecular species calculated using ab initio SCF MO methods. 33
Species State r(M.. .O)/pm r(0.. .H)/pm b/D
Be0 lc+ 132.1 -7.43
MgO lC+ 179.5 -a.44
(BeOH)+ IF.+ 134.0 94.4 -5.41
@@OH)+ 1c+ 170.9 94.5 -7.77
BeOH 1-f 1 139.9 93.4 0.85
MgOH 2c+ 177.4 94.1 -1.27
normal pressure phases of SrI2 (SrI2 -type structure) and BaI2 (PbC12-type structure) dissolve up to 20 mol.% BaI2 and 40 mol.%
SrI2, respectively, whereas the high pressure phases of Sr12 (PbCl2-type variant) and BaI2 (anti-Fe2P structure) dissolve up to 15 mol.% BaI2 and 60 mol.% SrI2, respectively.
Single crystal XRD studies have been undertaken for the hydrated alkaline earth metal halides, Ca12.4H20, 35
36 CaI2, 6.5H2035
and SrC12.6H20; whereas those of the calcium salts are novel investigations, 35 36 that of the strontium salt is a re-examination. The structure of CaI2,4H20 35 consists of centrosymmetric octahedral trans-configurated Ca12(H20)4 groups with r(Ca...I) = 312.7, r(Ca . ..O) = 229.9, 230.9pm. That of Ca12,6.5H20 35
contains binuclear Ca2(H20),34+ cations and I- anions; in the
cations, two Ca(H20)S square antiprisms are connected by two common triangular faces with r(Ca...O) = 239-260pm. In the
structure of SrC12,6H20,36 infinite chains of g-coordinate Sr atoms lie along the 3-fold axes of the trigonal P321 space group. Three water molecules are equatorially coordinated to each Sr atom, while the other six water molecules bridge to adjacent Sr atoms.
2.3.2 Ternary Derivatives The structural chemistry of several novel ternary oxides 37-43has
108
been elucidated using XRD methods; pertinent crystallographic
parameters are summarised in Table 4. Although the majority of
these polytypes were prepared by classical solid state methods,
CaTa206 and Ba3Lu409 were synthesised by Muller-Buschbaum 37,43
using the CO2 laser technique, and SrTa206 was obtained by Bayer
Table 4. Crystallographic parameters for diverse ternary oxides
and halides.
Compound symmetry Space a/pm b/pm c/pm e i O Ref Group
CaTa cubic Pm3 778 37
Cao.5Tao3 cubic Pm3m 388.9 _ 37
SrTa206 orthorhombic 1100.6 763.8 562.2 _ 38
Ba2TiO4 orthorhombic P21nb 610.7 2295.2 1054.0 - 39
Ba6Ti17040 monoclinic CZ/c 988.7 1709.7 1891.8 98.72 40
BaTi orthorhombic Pmmn 1452.7 379.4 629.3 - 40
Ba3Ru05 orthorhombic 888.0 449.0 688.0 - 41
Ba2RuO 4 tetragonal 398.1 1344.0 _ 41
BaRu03 hexagonal 574.5 2162.0 - 41 *
Ba4Ln207 tetragonal 436.6 2868.0 - 42
Ba3Ln307.5 t
tetragonal 438.6 1185.6 - 42
Ba3Lu409 trigonal R3 896 (a= 39.42"] - 43
Cs2BeC1 4 orthorhombic Pnma 964.2 717.8 1246.8 - 44
BaMnF5 orthorhombic p212121 1411.5 581.1 488.1 - 45
* Several isostructural compounds were studied (Ln = Gd-Lu,Y); the data refer to the yttrium derivative.
'Several isostructural compounds were studied (Ln = Ho-Lu,Y); the data refer to the yttrium derivative.
and Gruehn 38 both by deposition from the vapour phase using Cl2
as transporting agent and by crystallisation from B203 melts at
T<1423K. The three barium ruthenates(IV) were generated,
together with the Ba-rich compounds, Ba9RuOll and Ba4Ru0 6' for
which XRD data could not be indexed, by a group of Russian
authors 41 in a comprehensive study of the BaO-RuO2 system.
The structures of the novel MTa206 (M = Ca, 37 Sr38) polytypes
109
differ considerably. The calcium tantalate(V) 37 is a cubic modification which, although similar in structure to CaO
. 5TaO 3' differs in the presence of an ordered distribution of Cd atoms. The strontium tantalate(V) is a low temperature orthorhombic modification which reverts irreversibly to the normal structure at T>1493K.
1.r. and Raman spectra of M(V03)2 (M = Mg-Ba) have been recorded and assigned on the basis of structures by Fotiev et al. 44
Two ternary halides, Cs2BeC14 45 and structurally characterised; pertinent included in Table 4.
their monoclinic C2/m
BaMnF5, 46 have been unit cell parameters are
Optical visible absorption spectra of doped samples of CsMgX3 (X = C1,Br) have been measured by two independent groups. 47-49
Gudel et al have investigated V2"Clg5- dimer formation in CsMgC1347 II 5- and Mn2 X9 dimer formation in CsMgX (X = C1,Br)48 and McPherson et al have studied Cu M I IIIClg 5- (M = Cr,Mo,Ru,Rh) dimer formation in CsMgC13. 49 The absorption bands are assigned either to vanadium(II) or manganese(I1) pair excitations 47,48 or
to intermetallic Cu(I)+M(III) (M = Cr,Mo,Ru,Rh) charge transfer transitions. 49
Analysis 50 of the liquidus curve for the NaF-MgF2 system indicates that the melt probably contains MgF4 2- anions rather than MgF3- (or Mg 2+) ions, despite the fact that the only ternary compound formed in the system is NaMqF3.
2.4 COMPOUNDS OF THE ALKALINE EARTH METALS CONTAINING ORGANIC MOLECULES OR COMPLEX IONS.
Consideration of the papers abstracted for this section showed that a significant proportion belong to one of two subject groups
(viz., complexes of significance in bioinorqanic chemistry and complex formation in solution) which are common to several alkaline earth metals; these papers are considered in the appropriate subsection. The topics covered in the other papers are somewhat fragmented; these papers are discussed in subsections devoted to the individual alkaline earth metals.
2.4.1 Complexes of Significance in Bioinorganic Chemistry Diverse biologically active molecules are known to interact with
alkaline earth metals; those which have been studied in 1984,
110
however, fall into the three distinct categories Of
saccharides, 51-54 peptides 55,56 and nucleotides. 57-61
Complex formation between Ca 2+ ions and diverse
monosaccharides has been studied in solution both
spectroscopically 51 and calorimetrically. 52 The spectroscopic
evidence
bind Ca2"
indicates that those sugars with pyranose StrUCtUreS
ions strongly using three adjacent hydroxyl groups in
the ax-eq-ax configuration; for those sugars with furanose
structures, however, the spectra suggest that three cis hydroxyl
groups are required for strong bonding to Ca 2+ ion. The
calorimetric data 52 indicate that the behaviour of D-glucose and
D-fructose in the presence of Ca 2+ ions cannot be differentiated,
the free energy (-5.4(4) kJ.mol-I) and entropy (O(8) JK-'mol-'1
changes on formation of I:1 complexes being identical.
The thermodynamics of complex formation between the biopoly-
saccharide, heparin, and Ca 2+ ions have been determined 53 using
potentiometric techniques in aqueous solution adjusted to I = 0.3
by MC1 (M = Li,Na,K) or MgC12. When Ca*' ions are in excess,
complex formation was found to be dependent on the electrolyte
cation, either one (in the presence of Na ' or K+) or two (in the
presence of Li+, Na+ or Mg 2+) Ca2+ ions being coordinated to each
tetrasaccharide unit of heparin, When heparin is in excess,
however, Ca 2+ ion bridge formation linking two tetrasaccharide
units is also revealed in each system. 53
Complex formation between riboflavin and, inter alia, Ca 2+ ion
has been investigated 54 spectroscopically (i-r. and u.v.-visible)
in CH3CN with LiC104. 3H20 as supporting electrolyte. The spectra
indicate that coordination occurs through the pyrimidine carbonyl
and ribitol hydroxyl oxygen atoms.
Quantum mechanical calculations using the ab initio Hartree
Fock MO method have been completed for peptide bond formation in
the presence of Mg2+ ion55 and for complex formation between the dipeptides, glycylproline or prolylqlycine, and Ca 2+ ion.56 Two
mechanisms were examined for the SN2 reaction (i.e. two-step and
concerted) between qlycine and ammonia in the presence of Mg 2+
ion.55 The activation energies calculated for the Mg 2+ -catalysed
amide bond formation were substantially lower than those obtained
for the uncatalysed and amine-catalysed reactions. The catalytic effect of the Mg 2t ion is to stabilise both the
transition states and the intermediate; it is attributed to the
111
neutralisation of the developing negative charge on the electrophile and formation of a conformationally flexible non- polar five membered chelate ring structure (2).
Bond distances/pm Bond angles/" H \ Mg...O(l) 180.1 MgO(l)C(l) 117.7
O(2)
\ _ ,0(l) O(1) . . .C(l) 149.3 O(l)C(l)C(2~ 105.6
H C(l)_ - --_
--Mg C(1) . ..C(2) 158.1 C(l)C(2)N(l) 112.2
b’ \ H
\
/ /
C(2) . . .N(l) 152.2 C(2)Ntl)Mg 105.3
C(2) -N(l) N(1) . . .Mg 194.8 N(l)MgO(l) 89.9 H
I H
The stabilities of various geometrical forms of the complexes of Ca2+ion with both neutral (Zwitterionic) and anionic glycylproline and prolylglycine were determined to assess the different binding sites of these aliphatic dipeptides. 56 The configuration with the greatest stability was that in which the Ca 2+ ion is coordinated by both oxygen atoms of the carboxyl group. The structural chemistry of the binary compounds formed between
xanthosine-5' -monophosphate(S'-xmpmp) and Mg2+ ions,57 thymidine-5 '-monophosphate (5'-tmp) and Ca 2+ ions,58 deoxyadenosine-5 '-monophosphate (5'-damp) and Ca 2+ ions5' and of the ternary compounds formed between adenosine-5'-triphosphate
(5'-atp) , bis(2-pyridyl)amine (bipyam) and Mg 2+ or Ca2+ ions,60 has been elucidated using a variety of techniques. Spectro- scopic and chemical evidence obtained by Tajmir-Riahi and Theophanides 57 suggest that whereas the cation in Mg[5'-xmp],4H20 binds to the phosphate moiety as well as the N(7) atom of the purine ring, that in Mg[5'-xmp],9H20 only interacts with the N(7) atom of the purine ring; presumably the extra water molecu$es replace the phosphate moiety in the cation coordination sphere.
Sato has completed single crystal XRD studies of Cat5 '-tmpl,2H2058 and of Ca[5V-dampl,5H20.5g In the former structure5' the Ca2+ ion interacts with three phosphate oxygen atoms, two sugar oxygen atoms, one oxygen atom of the thymine base and a single water molecule; in the latter structure, 59 the Ca2+ ion interacts with four phosphate oxygen atoms and three water molecules. The Ca07 coordination spheres in both complexes
112
are irregular with fairly wide-ranging Ca-Q distances (229-l-
255.5pm in the 5 '-tmp derivative and 230.2-258.1pm in the 5'-damp
derivative).
The crystal and molecular structures of the ternary complexes
formed between 5'-atp, bipyam and M 2+ (M = Mg,Ca) have been
elucidated by Cini et al. fi0 They contain [M(H20)6J2+ cations,
[M(S'-Hatp)2J4- anions,uncoordinated Hbipyam+ cations and several
free water molecules and are best formulated as
[Mg(H20)6] [Hbipyam]2[Mg(5'-Hatp)2],12H20 and
[Ca(H20)61[Hbipyam]2[Ca(5'-Hatp)2],9H20. The [M(H20)612+ cations
have a distorted octahedral geometry; the metal atoms lie on a
2-fold axis with Mg-0 and Ca-0 distances averaging 212(6) and
240(7)pm, respectively.
Fiqure 1. Molecular structure of the [Mg(5'-Hatp)2]4- anion in
the [Mg(H20)6][Hbipyam]2[Mg(5'-Hatp)2],12H20 ternary
complex (reproduced by permission from J. Chem. Sot.,
Dalton Trans ., (1984)2467).
In the [M(S'-Hatp)2] 4- unit (Figure I), the metal atom has
octahedral coordination geometry and is Located on a 2-fold axis.
The two (5'-Hatp) 3- anions interact with the metal atom via the CL,
6, and Y phosphate oxygen atoms (Figure 1). The Mg06
coordination polyhedron is almost regular with an average Mg-0
distance of 206(2)pm; the CaO6 coordination polyhedron, however,
is a little more distorted with an average Ca-0 distance of
228(2)pm. No association occurs between the metal atoms and
113
either the nitrogen atoms of the purine system or the sugar oxygen atoms. 60
An ab initio MO theoretical study 61 of phosphate interaction with Mg 2+ predicts that the addition of two water molecules to t system does not affect the geometry of the magnesium phosphate complex. Implications for the association of alkaline earth metal cations with nucleotides and their derivatives are discussed. 61
:he
2.4.2 Complex Formation in Solution At least three complexes (BeL+,
detected62 Be(OH)L and BeL2) have been
by spectrophotometric and luminescence methods when Be2+ ions are treated with either 2-(o-hydroxyphenol)benzoxazole or 2-(o-hydroxyphenyl)benzothiazole. Where possible stability constants were calculated. 62 Stability constants have also been determined63 for complexes formed between dithiodiproprionic acid and methylenebis(thioacetic) acid and, inter alia, Be 2+ ions. The reaction between anhydrous MgBr2 and N-methylacetamide in
CC14 has been studied by i.r. spectroscopy; 64 the variation in the spectra as a function of time has been followed, the final spectrum being attributed to an addition product (2).
CH3 CH3 \ / CH-N
/ \ BrMg-0 Br
r--d NR2
cr a”, “, Y
NR2 (4)
The chelation ability and coordination modes of a series of 1,2- and 1,3-phenylenedioxydiacetamides for Ca 2+ ions have been systematically studied using spectroscopic (i-.x. 1 H- and 13C-nmr)
techniques. 65 Only the f,2-phenylenedioxydiacetamides (4) function as ligands; they act as tetradentate chelating ligands, forming 2:l complexes using all four oxygen atoms to bind the Ca 2+
ion.
2.4.3 Beryllium Derivatives Structural analyses have been completed on a number of diverse
beryllium derivatives. 66-72 The previously reported 73,74 "slip
114
sandwich" geometry of dicyclopentadienylberyllium (Figure 2) has
been confirmed in a redetermination 66 of its structure at 128K.
The Be atom is disordered between two equivalent sites in which
it is centrally (n5-) bonded to one ring and peripherally (nl-1
bonded to the other (Figure 2). Analysis of the molecular
geometry suggests that the peripherally bonded ring is attached
to the Be atom with a basically sp2 hybridised carbon. This
indicates only slight perturbation of the ring's delocalised
n-system and accounts for the recently reported 75 Raman spectrum
Figure 2. Lateral view of the structure of the beryllocene
molecule determined at 128K showing the "slip-
sandwich" structure (reproduced by permission from
Austral. J. Chem., 37(1984)1601).
which was thought to be inconsistent with an n'-C H ring.
5 g7 of
In an
independent i.r. solution (CS2, C6D,2, CDC13) study
dicyclopentadienylberyllium evidence is given for both ri5- and
ql-attachment of the C5H5 rings to the Be atom. For cyclopenta-
dienylberyllium chloride and bromide, ns-attachment of the C5H5
ring is confirmed using comparable i.r. data. 67
Electron diffraction data 68 for gaseous Be40(N03j6 at -430K are consistent with a T symmetry model for the molecule, the spatial
arrangement of atoms in gaseous Be40(N03j6 being generally very
similar to that in solid Be40(CH3C02)6. The principal
geometrical parameters refined to: r(Be...O(central ti4-oxygen
atom)) = 166.5pm, r(Be . ..O(nitrate oxygen atoms)) = 162.0pm,
115
r(N... O(coordinated oxygen atom)) = 129.8pm, r(N...O(terminal oxygen atom)) = 118.5pm, LONO = 117.0', q(the angle of rotation of the NO3 group about the 2-fold axis) = 25.2'.
Single crystal XRD studies have shown that whereas
[Be3C12(OButI ,I(21 69 [BeBr(OBut),Et2;~2(6),
70 is monomeric,
iBeH(Me2NCH2CH2NMe)12(1) and [Be(MeCzC)2,Me3N12(E) are dimeric.
In the unit cell of (5),6g the molecule lies at the intersection of two perpendicular mirror planes. The two chlorine and three
But But
(5-1
\ Br But 0 \ \ 'B,/"\B;('
7
LJ 'o'\ 0 But Br
L. (5)
Me
N Me3 C
Me 'Me
CMe 118-8M \ c \ 176.3 r& I ,NMe3
I ,’
Me
(a) distances/pm.
(b)
(8)
Be atoms lie along the intersection of these planes and are colinear. Adjacent Be atoms are bridged by two -OBut anions,
such that the Be(p-0But)2Be moieties are at right angles giving a
116
near tetrahedral coordination to the central Be atom, r(Be...O) =
164pm. The terminal Be atoms are trigonal planar, bonding to the
bridging oxygen atoms (r(Be.. .O) = 154pm) and terminal chlorine
atoms (r(Be...Cl) = 188pm). The shorter Be(sp2)-0 (154pm) than
Be(sp3)-0 (164pm) distances and the relatively short 3e-Cl
distances (typically 196pm) are indicative of the presence of
n-bonding around the 3-coordinate Be atoms. 69
In the presence of ether, the corresponding bromide forms a
dimeric structure (6) based on a centrosymmetric Be202 ring. 70 - The distorted tetrahedral Be coordination sphere is generated by
the two oxygen atoms of the bridging -OBut anions (l60,163pm), the
oxygen atom of the ether molecule (165pm) and a bromine atom
(219pm). The corresponding magnesium compound 76 has an analogous
structure with r(Mg...OBut) = 191,191, r(Mg.. .OEt2) = 201 and
r(Mg.. .Br) = 243.5pm.
The dimeric structure of (7)71 is similar; it is based on a
centrosymmetric Be2N2 framework; the Be atom achieves a
coordination number of 4 by virtue of the two bridging nitrogen
atoms (174.6,?74.7pm), a hydrogen atom (139pm) and the second
nitrogen atom of the amido ligand (181.4pm).
Crystals of (g)72 contain two independent centrosymmetric dimers
in which k-alkynyl groups exhibit quite different types of
interaction with the Be atoms. In (gab, the p-alkynyl groups
function as one electron donors leading to a predominantly
electron deficient Be2C2 ring in which significant cross-ring
Be-Be bonding occurs giving a short Be-Be contact (231.9pm). In
(gb) , the ring is effectively electron precise with the I-alkynyl
groups acting as three electron (u,~) donors with a longer
associated Be-Be distance (254.9pm). Pertinent geometrical data
for both dimers are included in the appropriate diagrams. 72
Reaction of I:1 molar ratios of BeC12 (or MgC12) and Ph3As=CH2
also yields 77 a dimeric beryllium(magnesium) species, in this
case a cation, [P~~AsCH~M(~~-C~~)MCH~A~P~~]~+ (M = Be,Mg) which is
based on a M2C12 (M = Be,Mq) framework. Although thermally stable, they are hygroscopic. Whereas the Be complex dissolves
readily in HN03 and HCl, the Mg complex is soluble in dmf, dmso,
CH30H and water;
CHC13.77 they are both slightly soluble in CH2C12 and
Vibrational (i.r. and Raman) spectroscopic studies 78 of alkaline
beryllate solutions have been effected; both aquo, [Be(H20)4]2',
117
and hydroxo, [Be(OH)412-, complexes were detected in solution. The two species were characterised by bands at 520cm -1 and in the range 870-900cm -' ([Be(H 0) 12+ 1 and by bands in the range 700-750cm 2-2 74 -' (IBe 1.
2.4.4 Magnesium Derivatives Since organomagnesium chemistry is reviewed elsewhere it is
generally ignored here. Consequently, of the plethera of papers published during 1984 which describe the chemistry of this element, but few have been abstracted for this subsection: those included, however, cover a diversity of interests with no obvious recurrent theme.
The reaction of Mg atoms, Mg2 dimers and Mg3 trimers with CH3Br in cryogenic (Ar) matrices has been studied 79 using u.v.-visible spectroscopic and matrix-gas replacement techniques. At high dilution Mg atoms are totally inert; Mg clusters, however, react by an oxidative-additive mode:
(Mg), + CH3Br + CH3(Mg)nBr . ..(2)
The higher reactivity of the Mg clusters is attributed to a greater thermodynamic stability of the cluster Grignard product, steric/orbital considerations and a lower ionisation potential of Mg clusters. 79
The amido complexes M2[Mg(NH2141 (M = K,Rb) have been prepared 80 by reaction of the metals with liquid ammonia at high temperature (423K) and pressure (200 MPa). XRD studies have shown that they are isotypic; single crystal data for K2[Mg(NH2j4] indicate that the Mg atom is tetrahedrally surrounded by four NH2- anions (r(Mg...N) = 204-209; r(Mg . ..H)>243pm] and that the two crystallographically distinct alkali metal atoms are located in rectangularly monocapped trigonal prisms afforded by NH2- anions
(r(K(l) . ..N) = 284.9-333.4; r(K(2)...N) = 300.5-331.9; r(K . ..H)>276pm). 80
Electron diffraction data for gaseous [MgiN(SiMe3]2}2181 at 400K
are consistent with a monomeric model (21. Significant structural features are the linear NMgN arrangement and the staggered Si2NMgNSi2 backbone. Minor torsional distortions lower the overall symmetry of the molecule from D2d to S4. 81
The cation, [Mg2C13,(thf)61 + (101, has been structurally
118
characterised by two independent groups; 82,83 a Dutch group 82
observed it in the product, [Mg2Cl,(thf)61f~TiC13(PhC~CPh)21-,
of the reaction of [TiC13(thf)3],diphenylacetylene and
Pr'MqCl in thf and a Polish group 83 found it in the product
[Mg2Cl,(thf)6]C[TiC1,(thf)]~ of the direct reaction of
distances/pm
r(Si.. .C) = 188.6pm
LNSiC = 116.4O
(9_)
(IO) -
[MqC12(thf)21 and [TiCl4(thf)2] in thf. The details of the two
structure determinations are very similar. The cation (IO) - contains two symmetry unrelated Mg atoms bridged by three chlorine
atoms, r(Mg(1) . ..Cl) = 248-251 (249.7-251.1); r(Mg(2]...C1) =
247-251 (251.2-251.5pm); each Mg atom has three terminal thf
molecules to complete their distorted octahedral coordination
spheres, r(Mg(l).. .o) = 207-210 (206.6-208.5); r(Mg(2)...0) =
204-211 (204-l-210.2pm). [Polish data83 are given in
parentheses].
The structures of two heterobimetallic complexes containing
magnesium have been elucidated. 84,85 Reaction of MgC12 with
FeC13 in thf yields [MgCl(thf)5l[FeCl41 which is reduced in the
presence of thf to form the Mg-Fe complex, tCl,Fe(p-C12)Mg(thf)4]
(11).84 - Crystallographic studies of (11) have shown it to -- contain pseudotetrahedral FeC14 units (r(Fe...Cl) = 222.8,237.7pm)
and pseudooctahedral MgC12(thf)4 units (r(Mg...Cl) = 250.lpm;
r(Mg . ..O) = 207.3,211.lpm) which edge share a pair of bridging
chlorine atoms. 84 Reaction of WOC14 with either the di.Grignard
reagent o-C~H~(CH~M~C~)~ or [Mq(CH2C6H4CH2-o) (thf)] in thf yields
either the trischelate [W(CH2C6H4CH2-~)3] or the Mg-W complex,
~Iw(cH~C~H~CH~ -o)2)(~-O]Mg(thf)4(/.t-O)tW(CH2C6H4CH2-oIl (z).85
Structural elucidation of (12) has shown the Mg atom to be - located at the inversion centre and to be octahedrally coordinated
by the two bridging oxygen atoms (207pm), and four thf oxygen
thf thf ,M/g/cl, /Cl
thE/ 1 \C1/FeiCl thf
(II) -
atoms (209,Zllpm). The five-coordinate W atom environment is highly distorted owing to a very short W-O bridging contact (171pm) and the restrictions associated with the presence of two chelating ligands (r(W...C) = 214-218pm). 85
Hydrated alkaline earth metal (Mg-Ba) salts of 86 and HOCH[CH2N(CH2P03H2)2]2 87
(CH2COOH)2 have been synthesised and their thermal
properties ascertained using dta, tga and XRD methods. The carboxylates, M(CH2C00)2,nH20 (Mg, n=4; Ca, n=1.5) decompose via the anhydrous salt and the carbonate to the oxide; the decomposition of the anhydrous salts M(CH2C00)2 (M = Sr,Ba), however, proceeds no further than the carbonate. 86 The phosphates, M4(HOCH[CH2N(CH2P03)2]21 (Mg, n=8; Ca, n=4; Sr, n=6; Ba, n=81 dehydrate in either a single stage (Mg,Ca,Sr) or a two stage (Ha) process eventually forming the anhydrous salts. Further decomposition was not studied. 87
The structure of magnesium bis(hydrogen maleate)hexahydrate has been ascertained by single crystal. methods. 88 The Mg atom is located at an inversion centre and is coordinated by a slightly distorted octahedron of water molecules (203.6-205.9pm); bonding to the carboxylate anion is through the water molecules via strong hydrogen bonds. 88
The magnesium-anthracene (l/l) complex crystallises from thf with three solvate molecules. 89 Multinuclear nmr ('H, 13C)
120
studies8' indicate that, in thf solution, magnesium, anthracene
and the complex exist in a temperature dependent, reversible
equilibrium:
Mg f . . (3)
the formation of the complex being favoured at lower temperatures.
The data also show that the Mg atom interacts most strongly with
the 9,10-positions of the aromatic ring system. 89 Reaction of
the complex with dialkyl aluminium hydrides in thf yields
magnesium k-(9,10-dihydro-9,10-anthrylene)-dialkylhydrido-
aluminates; 90 similar products are obtained with aluminium
trihydride and with ethoxydiethylaluminium. The structures of
these compounds when dissolved in thf were determined by nmr
spectroscopy. 90 An XRD study 90 of the diethylhydridoaluminate
(Figure 3) showed that Mg and Al occupy axial positions in a
9,10-dihydro-9,10-anthrylene system, r(Mq...C) = 219.5, r(Al...C)
= 203.6pm; and are bridged by the hydrogen atom, r(Mg...H) = 196,
r(A1 . ..H) = 162pm. The five-fold coordination polyhedron of the
Mg atom (Figure 3) is completed by the oxygen atoms of three thf
molecules (205.8-210.6pm) and the four-fold coordination sphere
of the Al atom (Figure 3) is completed by the carbon atoms of the
two ethyl moieties (198.0;198.4pm). 90
2.4.5 Calcium Derivatives
A total of eleven papers have been abstracted for this
subsection. They all describe the results of single crystal XRD
studies; eight carboxylates, 91-97 one sulphonate, 98 and three
other more diverse materials 99-101 have been studied, Pertinent
features of the Ca atom coordination polyhedra in the organic
saltsg1-g8 are summarised in Table 5. The Ca07 coordination
sphere in Ca(HP03H)2,H20 99 is generated by six oxygen atoms from
phosphate anions (average r(Ca . ..O) = 241.3pm) and a single water
molecule (r(Ca . ..O) = 237.7pm) arranged in a distorted mono-
capped triqonal prismatic geometry. A seven-coordinate Ca atom
is also found in the structure of CaT12Cl10,7H20; 100 each Ca atom is surrounded by seven water molecules (average r(Ca...O) = 240.1
pm) disposed in a rectangularly capped trigonal prismatic geometry.
In the structure of the CaC12-sarcosine (l/3) adduct, 101 the Ca
121
Figure 3. Molecular structure of magnesium-L-(9,10-dihydro-9,10- anthrylene)-diethylhydridoaluminate (reproduced by permission from Chem. Ber., 117(1984)389).
atom is located in a slightly distorted CaO6 octahedral unit, the oxygen atoms being provided by six separate sarcosine molecules (CH3NHCH2COO~), r(Ca...O) = 229.4-238.7pm. Temperature dependent studies have shown that (i) the mirror symmetry of the paraelectric phase (>127K) is lost in the ferroelectric phase (<127K) and (ii) some disordered motion such as a reorientational motion of the Ca06 octahedra occurs in the paraelectric phase. 101
2.4.6 Strontium and Barium Derivatives The traditional dearth of papers for this subsection has been
maintained. Only four papers have been abstracted; they describe the crystal and molecular structures of one strontium 102
and three barium 103-105 derivatives. The structure of strontium digold octaacetate dihydrate lo2 consists of isolated SrAu2(CH3C00)8 units (13) with crystallographic symmetry 2. The coordination - geometry is square planar at Au (r(Au . ..O) = 197.4-199.2pm) and square antiprismatic at Sr (r(Sr...O) = 254.8-259.6pm). The C-O bond lengths indicate covalent Au-O but electrostatic Sr-0 interactions. 102
Poonia et al 103-105 have reported structural data for the three barium derivatives, tetraaquabis(l,lO-phenanthroline)barium(II)
Tabl
e 5.
Pertinent
features of th
e Ca
at
om c
oord
inat
ion
poly
hedr
a in
var
ious
ca
rbox
ylat
e 91
-97
and
sulphonate 98
salt
s.
Salt
Co
ordi
nati
on*
Poly
hedr
on
r(Ca
...O
) Re
f
/:p LI
-c%p
-C
//O
\ a+
H2
°
Ca(C
H3CO
0)2,
H20
Ca(l
)07-
D5h
2; 24
6.0,
249-
O 3;
230
.1-2
50.3
2;
23
6.0,
236.
8 91
Ca(2
)08-
irre
gula
r 4;
23
9.9-
293.
3 4;
22
7.0-
249.
6
CaH(
CH3C
OO)2
,H20
Ca
(l)0
7-D5
h 2;
25
1-O-
252.
4 4;
23
5.2-
242.
4 1;
237
.5
92
Ca(2
108-
D2d
4; 25
1.0-
259.
4 3;
234
.7-2
38.8
1;
237
.2
Ca2B
aI(C
H2)2
CHCO
O]6
CaO6
-Djd
22
8.9+
93
Ba01
2 30
0.8+
Ca2B
a[(C
H3)2
CHCO
O]6
Ca06
-D3d
22
5.3+
93
Ba01
2 29
6.5+
Ca[H
C(OH
)C00
]2,3
H20
Ca07
-irr
egul
ar
4; 22
1.4-
241.
9 1;
239
.6
2; 23
6.1,
238.
4 94
p
I Ca
2tCH
(COo
){cH
(CoO
13)0
1,2H
20
Ca(l
)08-
irre
gula
r 2x
lx
3x
95
Ca(2
)08-
irre
gula
r 2x
lx
3x
Ca[2
-FC6
H4C0
0]2,
2H20
Ca
08-D
2d
4; 24
6.3-
262.
9 2;
23
6.0
2x
96
Ca[&
-OH-
3,6-
C~-C
6H2C
OO]2
,2H2
0 Ca
08-D
4d
6; 25
1.6-
251.
7 2;
23
7.2
2; 24
1.4
97
Ca[4
-NH2
C10H
6S03
12,8
H20
Ca07
-D5h
7;
23
7.5-
243.
5 98
* D
5h-
dist
orte
d pe
ntag
onal
bi
pyra
mid;
D4
d -d
isto
rted
sq
uare
an
tipx
ism;
D3
d -d
isto
rted
tr
igon
al an
tipr
ism;
D2
d-di
stor
ted
dode
cahe
dral
. t Av
erag
e va
lues
.
x Bo
nd di
stan
ces
are
avai
labl
e as
su
pple
ment
ary
mate
rial
.
N
C-J
123
The Au04 rings are rotated by 45' relative to each other to give Dqd symmetry at the Sr atom.
(13) -
(-+? O3 bis(2,4-dinitrophenolato)bis(triethanolamine)barium(II) (G)'04 and bis(2-nitrophenolato)barium(II) (s).'05 Markedly different coordination polyhedra are found in the three compounds; that in (14) is a distorted cubic Ba04N4 arrangement, that in (15) - is a monocapped distorted cubic Ba07N2 geometry, and that in (16) is a distorted one side bicapped square antiprismatic BaOIO configuration. The Ba atom in (14)'03 is located at the - inversion centre of the Pi space group; it is surrounded by four nitrogen atoms from two l,lO-phenanthroline ligands (294.0,294.5 pm) and four oxygen atoms from water molecules (276.3,277.8pm). The Ba atom in (15)lo4 - is bonded to the eight heteroatoms of the two triethanolamine ligands (r(Ba... 0) = 270.3-289.4; r(Ba...N) = 300.0,303.6pm) and to a single oxygen atom of one of the dinitrophenolate anions (278.5pm). The Ba atom in (16)lo5 lies - on a 2-fold axis and is coordinated to four phenolic (271.1,272.2 pm) and six nitro (290.0-317.7pm) oxygen atoms.
124
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